System for receiving sounds in the presence of disturbing noises



June 2%, 1948. L" ,J s v 2,444,069

sYs'rEu FOR RECEIVING SOUNDS IN THE PRESENCE OF DISTURBING NOISES Fileq March 2, 1945 FIG.

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DIRECT/0N 0f MfiX/Ml/M Fi s/@3555 I'D/P HVD/MHIMVL' ll FIG. 2 l2 PHME SHIFT l0 OUTPUT IN l/E/V TOR L Ji 5/ VIA [V ATTORNEY Patented June 29, 1948 SYSTEM FOR RECEIVING SOUNDS IN THE ZRESENCE 0F DI STURBING NOISES Leon .3. Sivian, Summit, N. 3., assigncr to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application March 2, 1945, Serial No. 580,684

This invention relates to signal translating sys- I tems and more particularly to submarine signaling systems especially suitable for the reception of low frequency signals.

In some situations it is desirable to view sonically a wide area, for example a 360-degree field about a reference point, in the presence of a nearby source of sonic signals. For example, in some signaling systems on ships, it is desirable that submarine signals emanating from a distant source at any direction with respect to the ship be detectable at the ship. The propeller of the ship, however, constitutes a disturbing noise source which may interfere seriously with the detection of the desired signals.

One object of this invention is to enable the reception at a station of signals emanating from any point within a wide angular field with reference to the station, in the presence of a disturbing signal source in the vicinity of the station. More specifically, one object of this invention is to substantially eliminate the effects of propeller noise upon the reception at a ship of submarine signals emanating from a distant source.

In accordance with one feature of this invention, a signal translating system comprises a pair of signal responsive devices constructed and associated so that the resultant of the combined outputs of the two devices for signals emanating from a source in the vicinity thereof is substan tially zero, whereas for signals emanating from a distant source the resultant is substantially equal to the output of one of the devices.

In a specific and illustrative embodiment of this invention, a submarine signaling system suitable for use on a ship comprises a pressure-type hydrophone and a pressure gradient-type hydrophone and means for adjusting the relative phase and amplitude of the outputs of the two hydrophones so that for noise originating at a propeller of the ship the resultant of the combined outputs of the two hydrophones is substantially zero whereas for signals emanating from a source distant from the ship, the resultant of these outputs is substantially that of the pressure-type hydrophone.

The invention and the above-noted and other features thereof will be understood more clearly and fully from the following detailed description with reference to the accompanying drawing in which:

Fig. 1 is a diagram which will be referred to hereinafter in a discussion of certain principles involved in this invention; and

'aso

Fig. 2 is a circuit schematic showing the electrical association of the signal translating devices in an illustrative embodiment of this invention.

The principles involved in submarine-signal detecting or receiving systems constructed in accordance with this invention will be understood from the following consideration with reference to Fig. 1. In this figure, l0 designates a nondirectionai hydrophone of the pressure type and H designates a directional, velocity or pressure gradient-type hydrophone, the two hydrophones being mounted in proximity, for example in vertical alignment below the keel of a ship, and so thathbothare exposed to signals emanating from rce anywhere within a 360-degree field about a. prescribed axis, for example the vertical axis of alignment of the two hydrophones. Such a source, which may be a submarine, is indicated in Fig. 1 at S, a distance To from the hydrophones. A disturbing nearby source, for example aship's propeller, is indicated at $1, at a distance 11 from the hydrophones, the distance 11 being much smaller than the distance To and, for reasons which will be appreciated from subsequent discussion, is small in comparison to the wavelength of the highest frequency it is desired to detect or receive.

The output of the hydrophone ID in response to signal waves received thereby is independent of the angle of incidence of such waves thereon. That is, this hydrophone is non-directional or omnidirectional. This output is proportional to the pressure of the incident waves and the pressure, as is known, varies inversely as the distance between the hydrophone and the signal source;

The output of the hydrophone H is dependent upon the angle of incidence of the signal waves and is proportional to the pressure gradient of such waves. The pressure gradient, as is known, comprises two factors or components, one of which varies inversely as the distance noted and the other of which varies inversely as the square of this distance. These components may be in or out of phase depending upon the character of the wave front at the hydrophone. Specifically,

for distances large in comparison to the, wavelengths of the signals received, the wave fronts are essentially plane and the two components noted are substantially in Phase; for distances small in comparison to the. wavelength, the wave fronts are highly curved and the two components are out of phase to an extentdependent upon the distance.

The outputs of the. two hydrophones may be expressed mathematically as:

E,.,=B.,% sin hr-la 1 and where E1o=output voltage of the hydrophone l En=output voltage of the hydrophone i i Blo=a constant determined by the sensitivity of the hydrophone i0 Bn=a constant determined by the sensitivity of from this source, yet produce a substantial output, essentially that of the hydrophone II, for

the hydrophone ii 7 W r=distance from the sound source to the hydrophones =21 times the frequency of the signals t=time =the angle between the direction of the incident signals and the direction for which the hydrophone H has the maximum response k=a constan I c=ve1ocity of the signals, e. g. velocity of sound in water 9=tankr=the phase angle of the resultant of the two components of the pressure gradient- It will be noted from Equations 1 and 2 that a resultant output (Em+En) of substantially zero from the two microphones can be obtained for signals emanating from a source at any fixed 86 distance from the hydrophones by correlation of the sensitivities of the two hydrophones and the phases of the output voltages.

advantageously, the hydrophone I I is so mounted relative to the disturbing source S1 that the anan gle o is zero. Then, the output of the two hydrophones in combination for signals emanating from the source 31 can be made zero by adjusting B o or Bn or both so that and retarding the phase of E by 01=tan (kr1) (4) The adjustment of B10 and B11 may be efiected, w

Em=Bm; sin (km-wI-BQ '1 As noted heretofore, the distance 11 is very small in comparison to the wavelength of the signals received. For a source S at a substantial dis- En: B,,(eos sin [kr-wl-Ol) a parison to the wavelength. The term 70 also is very small for audio frequencies. As will be aP- parent, the ratio then, is very small and, therefore, En as expressed by Equation 6 also is very small. Hence,

Thus the hydrophones eifectively discriminate against the disturbing source S1, 1. e), produce substantially zero output for signals emanating signals emanating from a distant source S.

Theoretically, perfect discrimination against the disturbing source S1, 1. e., zero resultant output for signals from the source, obtains only for a single frequency inasmuch as both the sensitivity terms Bm and Bu and the phase angle a involve the frequency. Practically, however, satisfactory discrimination over a band of frequencies is achieved by proper positioning of the hydrophones relative to the disturbing source 81. For example, discrimination of at least decibels over a given frequency range can be realized by mounting the hydrophones so that n is equal to or less than 120 of the wavelength of the highest frequency in the range given. For such value of n, the phase adjustment required may be expressed as duced by the hydrophones and the associated cir-- cult. In some hydrophones, the relation in amplitude and phase, of the outputs of hydrophones i0 and II to the pressure and pressure gradient respectively of the incident sound waves may vary differently with frequency. Advantageously, in order to simplify the adjustments I requisite to discriminate against the disturbing source $1, thehydrophones are selected so that each exhibits a single form of vibrational control throughout the desired frequency range. For example, the pressure hydrophone in may be stiffness-controlled over the desired frequency range and the pressure gradient hydrophone may be mass-controlled over this range, so that but a single adjustment of relative amplitude and phase provides substantial discrimination against the source S1 over the desired frequency range.

What is claimed is:

1. A signal translating system for discriminating between signals emanating from a pair of widely spaced sources, said system comprising a pressure-type signal transducer, a pressure gradient-type signal transducer, said transducers being positioned in proximity to one of said sources and in fixed relation thereto, means for fi n e fr m the h ph is l rge in comto combining the outputs of said transducers to produ-ce a resultant determined by the difference of said outputs, and means for adjusting the relative phase and amplitude of said outputs so that said resultant for signals emanating from said one source is substantially zero, said pressure-type transducer being spaced from said one source a distance greater than that between said one source and said pressure gradient-type transducer and both said transducers being spaced from said one source a distance comparable to a small fraction of the wavelength of a preassigned frequency to be translated by said system.

2. A sonic signal translating system for detecting signals within a preassigned frequency range emanating from a distant source in the presence of signals of similar character emanating from a nearby source, said system comprising a nondirectional pressure-type signal transducer, a pressure gradient-type signal transducer, said transducers being spaced from said nearby source a distance small in comparison to the wavelength of the highest frequency in said preassigned range, means for combining the outputs of said transducers to produce a resultant determined by the difierence of said outputs, and means for adjusting the relative amplitudes of said outputs and shifting the phase of the output of said pressure-type transducer so that for signals emanating from said nearby source said resultant is substantially zero.

3. A signal translating system in accordance with claim 2 wherein said pressure-type transducer is stifiness-controlled throughout said range and said pressure gradient-type transducer is mass-controlled throughout said range.

4, A submarine signal translating system for detecting signals within a preassigned frequency range emanating from one source, in the presence of a second source remote from said one source, said system comprising a non-directional pressure, stillness-controlled hydrophone, a pressure gradient, mass-controlled hydrophone, said hydrophones being spaced from said second source a distance small in comparison to the wavelength of the highest frequency within said range and said pressure hydrophone being spaced from said second source a distance greater than that between said pressure gradient hydrophone and said second source, means for combining the outputs of said hydrophones to produce a resultant determined by the difierence of said outputs, and means for adjusting the relative amplitude and phase of said outputs so that for signals emanating from said second source said resultant is substantially zero.

LEON J. SIVIAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,312,809 Scribner et a1 Aug. 12, 1919 2,368,953 Walsh Feb. 6, 1945 2,376,730 Steinhoff May 22, 1945 FOREIGN PATENTS Number Country Date 297,977 Germany Jan. 19, 1920 527,678 Germany Sept. 23, 1931 567,999 Germany Jan. 12, 1933 

